It depends on what version. 97-2003 use .doc
I don't know any others
Excel 2007 creates xlsx files, and earlier versions of Excel create xls files. The earlier versions won't be able to open the 2007 files. So you need Excel 2007 to open them or get a converter or try and open them in Google docs. If you get them open, then save them in your version of Excel so that they can be opened in future, using the Save As option on the File menu.
Download FileFormatConverters from Microsoft. This program will allow you to pen docx files with earlier versions of Microsoft Office.
They're used as extensions to identify picture files.
Backup and Restore Utility or by System Protection
The computing term "backward compatibility" refers to the ability of a model, software, or other computer object that can work properly with earlier models and versions. Many PDF files for example are compatible with earlier versions of PDF reader.
It depends on what version. 97-2003 use .doc I don't know any others
No. WMP is just the program that your system uses to open song files, upgrading it won't affect the files themselves.
No, folders usually do not have file extensions, but they could have if you like. Usually only files have extensions, to indicate the contents of the file.
.jpeg, .bmp, .png
MP3 sound files have an .mp3 file extension, and common file extensions for raw, uncompressed sound files are .wav and .aif. Pg. 474
No The files are programmed applications with extensions whereas the folders is like a wallet u keep the files in it
AutoCAD is a drafting and design program used in many industries. You can create drawings, blueprints, solid models, etc... Creates files with extensions such as .dwg, .dxf.